FIG. 1 depicts a typical arrangement for a conventional geostationary earth orbit (“GEO”) communications spacecraft. Normally, the yaw axis of a conventional spacecraft (such as conventional spacecraft 100) is directed at Earth's center, the yaw/roll plane is parallel to the equatorial plane, solar arrays 102 rotate about the pitch axis and orbit normal, and transponder panels 104 face in the north/south direction. Communications antennas 105 remain fixed, pointed at their Earth coverage regions.
As the angle from the sun to the spacecraft's yaw/roll plane increases, the solar radiation input to the spacecraft's heat-rejecting transponder panels increases, reducing the spacecraft's ability to reject waste heat. The increase in the sun's angle also increases the angle between the solar panels and the sun vector, thereby reducing the power produced by the solar arrays. Both of these effects potentially limit the payload that a spacecraft can carry. The typical arrangement depicted in FIG. 1 provides a conventional GEO communication spacecraft with adequate solar array power and thermal heat rejection, because it limits the angle between the sun and the spacecraft's yaw/roll plane to within the range of sun declination angles, or ±23.5°.
Despite its elegance and simplicity, a problem arises when conventional GEO spacecraft, such as spacecraft 100, are adapted to highly inclined orbits of the type presently in use to provide LMSS. For example, the SIRIUS SATELLITE RADIO® mobile satellite system provides S-band Digital Audio Radio Service (“S-DARS”) using three satellites in 24-hour period repeating ground track orbits with inclination of 63.4 degrees, eccentricity of 0.27, argument of perigee of 270°, and apogee longitude of 96° west. Using the FIG. 1 spacecraft arrangement, the maximum yaw/roll plane sun angle, or the sum of the maximum sun declination and the orbit inclination, would be 87°, resulting in insufficient power and heat rejection to operate a communications payload.
To address this problem, the SIRIUS SATELLITE RADIO® mobile satellite system satellites use a sun nadir pointing (“SNP”) yaw steering approach, rather than the conventional approach outlined above. See, e.g., R. Briskman et al., S-DARS Broadcast From Inclined, Elliptical Orbits, 52nd International Astronautical Congress, October 1 to 5, Toulouse, France. Using SNP, the yaw axis of the spacecraft remains pointed to Earth, while the spacecraft continuously rotates about the yaw axis to keep the sun vector in the spacecraft's yaw/roll plane. Although this strategy provides beneficial power and thermal conditions using a conventional GEO spacecraft, SNP requires that the spacecraft be capable of executing large-angle yaw rotations. FIG. 2, which is a chart depicting the required yaw rotation for SNP, illustrates that a 160° rotation is required for a sun beta angle of 10°.
Further to these disadvantages, SNP also restricts the antenna designs that can be utilized to specific types which can compensate for the spacecraft yaw rotation. For example, the SIRIUS SATELLITE RADIO® mobile satellite system S-band antenna utilizes a folded optics design that provides elliptically shaped coverage for North America. As the spacecraft rotates in yaw to follow the yaw steering profile, the antenna sub-reflector counter-rotates correspondingly, so that the elliptical pattern remains properly oriented over the coverage region.
This known rotating sub-reflector approach is not feasible for the next generation of the antennas envisioned for LMSS. The next-generation antennas may have large reflector surfaces (e.g., 12 meters compared to 2.5 meters for the conventional systems), which are contoured to provide a coverage pattern that closely matches the shape of the continental United States. The next generation approach will increase the signal directivity by up to 2 to 3 dB, increasing the quality of service, and increasing the probability that the signal strength will be sufficient for mobile receivers to operate.
Commonly-assigned U.S. Pat. No. 6,616,104 (“the Cheng patent”) describes an improved arrangement for a spacecraft which remedies some of the deficiencies of other conventional spacecraft. The Cheng patent describes a system which utilizes body steering only, fixing the antenna boresight in the spacecraft body frame, with a roll angle equal to the orbit inclination. With a fixed antenna boresight, the arrangement described in the Cheng patent is generally limited to spacecraft which operate in a narrow region about orbit apogee, such as spacecraft which operate over a limited range of latitudes and only 8 hours of a 24-hour orbit period.
Due to their substantial build, launch and operational costs, it is considered highly desirable to overcome the deficiencies of conventional LMSS spacecraft configurations and attitude steering methods. Specifically, it is desirable to provide an enhanced land mobile satellite configuration and steering method which provides a favorable sun-spacecraft geometry compatible with current and future payload power and thermal requirements, without the need for yaw steering.